1. Field of the Invention
This invention relates to radiation sensors in general and more particularly to a radiation detection panel comprising a plurality of radiation sensors each associated with a protective and addressing diode.
2. Description of Related Art
Radiation sensors able to convert incident radiation directly to an electrical charge indicative of the intensity of the incident radiation are known. Typically such sensors comprise a complex structure which includes a bottom and a middle conductive electrode separated by a dielectric to form a capacitor, a photoconductive layer over one of the electrodes and a top electrode over the photoconductive layer to apply a charging voltage to the sensor. The structure thus forms two capacitors connected in series. A voltage is applied between the bottom and top electrodes charging the two capacitors. Upon exposure to radiation the photoconductor becomes conductive altering the charge distribution in the two capacitors. With proper selection of electronics and materials this results in a charge accumulated and stored in the capacitor formed by the bottom and middle electrodes which is proportional to the exposure to the incident radiation.
Read-out of the stored charge is usually done by addressing the middle electrode and flowing the accumulated charge to a charge measuring device such as a charge integrating amplifier.
A plurality of such sensors may be assembled in an array of rows and columns to form a radiation detection panel. By sequentially reading out the charges accumulated in the individual sensors an image of the relative exposure of different areas of the panel is obtained. This image represents the radiation intensity incident on the panel after it has passed through a subject illuminated by the radiation. When the radiation is X-ray radiation and the subject is a patient the resulting image is a radiogram, captured as a plurality of charges. This radiogram can be displayed on a Cathode ray tube or other device for viewing.
The charge stored in the capacitor is read-out using a switch which connects, upon command, the middle electrode to the input of the charge measuring device. In practice such switch is usually an FET transistor created integrally with the sensor, having its drain connected directly to the middle electrode of the sensor. Both the source and the gate are accessible from outside the sensor. The source is connected to the charge integrator. A pulse applied to the gate switches the transistor to a conductive state and permits the charge to flow from the capacitor to the integrator for detection.
The above described technology is well known in the art and well described in a number of publications and issued patents, exemplary of which is U.S. Pat. No. 5,319,206 issued Jun. 7, 1994 to Lee et al., and hereby incorporated by reference herein.
When an array of sensors is used, there is need to have a plurality of connecting conductive lines extending between the sensors interconnecting the gates, and sources of the FETs so that the FETs may be addressed and the signal from the capacitors retrieved. The use of a transistor as a switching device necessitates two lines, one for addressing the transistor to turn it on/off and the second for flowing the charge to the integrator.
The simplified sensor and transistor structure described above while quite adequate is, however, vulnerable to overexposure. The term "exposure" is used in this specification to designate the product of the intensity of the incident radiation times the time during which the radiation impinges on the sensor. As the exposure increases, the charge build-up on the charge accumulating capacitor, and consequently the voltage on the middle electrode also increases, eventually exceeding the operating limits of the transistor and destroying it.
One solution to this problem is the provision of a thick, charge blocking layer, interposed between the photoconductor and the top electrode, which in effect prevent the catastrophic charge build up in the charge accumulating capacitor.
While this is an acceptable solution to the charge build up problem, the presence of the thick blocking layers requires that the sensor, following exposure and read-out of the accumulated charge in the charge accumulating capacitor, be further discharged prior to the next exposure. This extra step is not only time consuming, but inhibits the use of such type of sensors for continuous, real time imaging, such as in fluoroscopy applications.
It is an object, therefore, of the present invention to provide a sensor which is protected from catastrophic failure due to overexposure, and an associated panel comprising a plurality of such sensors, where the lines leading to the sensors are minimized and which has fast response for use in real time viewing applications.